Ballooning costs

During the 2008 financial crisis, Washington put Freddie Mac and Fannie Mae into "conservatorship" - a quasi-nationalised status in which the federal government promised to maintain their solvency by injecting new equity on demand.

Originally, the two home loan agencies' total needs were estimated to be $200bn - of which three-quarters has already been provided - but the FHFA has now revised that figure upwards.

The regulator's upper estimate of $363bn would only happen if there is a second downturn in the US housing market - something that many economists believe has already begun since the expiry in April of a tax credit for homebuyers.

Litigation risk

Freddie Mac and Fannie Mae bought up billions of dollars of mortgages from the big banks, then repackaged them and sold them to investors with their own guarantee.

The FHFA now says that the banks may have breached promises they made in the original sale agreements about the quality of the loans they were selling.